Reluctance transducer

ABSTRACT

A reluctance transducer includes a soft ferromagnetic yoke and a soft ferromagnetic core element, which is movable relative to the yoke. Two permanent magnets bear the core element. The permanent magnets are arranged relative to each other and to the yoke so that the reluctance transducer has a good linear relationship between displacement and force. The reluctance transducer can be applied as stiffness compensating element. The reluctance transducer can include an electrical winding to allow its application as a magnetic bearing, an actuator or as a displacement, velocity or acceleration sensor with improved intrinsic linearity.

The invention relates to a reluctance transducer comprising a softferromagnetic yoke and a soft ferromagnetic core element being movablerelative to each other and further comprising two permanent magnets.More in particular the invention relates to such a reluctance transducercomprising a soft ferromagnetic yoke having a first end and a secondend, the first end and the second end defining an intermediate space,and comprising a soft ferromagnetic core element partly filling theintermediate space, the core element and yoke being movable relative toeach other in a direction between the first end of the yoke and thesecond end of the yoke and further comprising a first permanent magnetand a second permanent magnet arranged relative to each other such thatthe permanent magnets exert opposite magnetic forces on the coreelement.

STATE OF THE ART

Reluctance transducers are applied either as passive transducer or asactive transducer. Passive reluctance transducers are used as stiffnesscompensating element. Active reluctance transducers are implemented forapplication as an actuator, as a sensor or as a magnetic bearing. Suchactive transducers comprise an electrical winding to generate a magneticflux in the soft magnetic yoke. The position of the core elementrelative to the yoke can be influenced by varying the current in thewinding which allows the application as an actuator. The reluctancetransducer can also be applied as a sensor whereas moving the coreelement relative to the yoke results in an electrical current in thewinding.

International application WO98/37335 discloses a magnetic bearing anddrive. This known reluctance transducer, as shown in FIG. 1, comprisestwo permanent magnets that are clamped between two magnetic yokes. Amovable core element is partly filling the space between the ends of theyokes. A portion of each of the yokes is wound with an electricalwinding. A disadvantage of this know reluctance transducer is that it israther complex and that it is limited in the number of differentembodiments that can be realised.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a reluctancetransducer with improved performance, more in particular with a betterintrinsic linearity.

This objective of the invention is obtained by a reluctance transducercomprising

-   -   a soft ferromagnetic yoke having a first end and a second end,        the first end and the second end defining an intermediate space,    -   a soft ferromagnetic core element partly filling the        intermediate space, the core element and the yoke being movable        relative to each other in a direction between the first end of        the yoke and the second end of the yoke,    -   a first permanent magnet and a second permanent magnet arranged        relative to each other such that the first magnet and the second        magnet exert opposite forces on the core element, and        wherein a first pole of the first magnet is mechanically and        magnetically coupled by an intermediate soft ferromagnetic        element to an equivalent pole of the second magnet,        characterized in that a second pole of the first magnet is        magnetically insulated from an equivalent second pole of the        second magnet.

An advantage of coupling only one set of equivalent magnet poles, forexample the South poles, mechanically and magnetically with each othervia a soft ferromagnetic element is that there are mainly just twomagnetic flux circuits for each of the permanent magnets. This meansthat there are two main paths outside each magnet along which themagnetic field lines are closed. When displacing the core elementrelative to the yoke in a direction towards one of the two ends, infirst order approximation the magnetic resistance of only one of themagnetic circuits varies. The effect of such a magnetic circuit is thatthe relationship between the displacement of the core element and theforce exerted on that element is better linear than in the knowntransducers, whereas the design of the transducer is less complex. Thiseffect results in a reluctance transducer with improved performance anda less complex design. More in particular it results in a reluctancetransducer that is intrinsically better linear than known reluctancetransducers, without the need of electronic control measures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a reluctance transducer according to the state of the art;

FIG. 2 shows a schematic drawing of an embodiment of the reluctancetransducer that can be applied for stiffness compensation;

FIG. 3 shows an embodiment of the reluctance transducer that can beapplied as a magnetic bearing, an actuator, or sensor;

FIG. 4 shows an embodiment of the reluctance transducer wherein themagnets are arranged within the yoke;

FIG. 5 shows a cross-sectional view of a preferred embodiment of thereluctance transducer that can be applied as an actuator, as a sensor,or as magnetic bearing;

FIG. 6 shows an embodiment of the reluctance transducer in which themagnets are fixed to the core element;

FIG. 7 shows an embodiment of the reluctance transducer in which themagnets are arranged perpendicular to the line between the two ends ofthe yoke;

FIG. 8 shows an embodiment of the reluctance transduces in which themagnets are arranged within the yoke, parallel to the line between thetwo ends of the yoke;

FIG. 9 shows a schematic perspective view of an actuator structure;

FIG. 10 shows a schematic exploded view of an actuator from the actuatorstructure shown in FIG. 9;

FIG. 11 shows a schematic side view of a details of the actuator shownin FIG. 10;

FIG. 12 shows a schematic perspective view of a first embodiment of amagnetic bearing;

FIG. 13 shows a schematic cross-sectional view of the magnetic bearingshown in FIG. 12;

FIG. 14 shows a schematic perspective view of another embodiment of amagnetic bearing; and

FIG. 15 shows a schematic exploded view of the magnetic bearing shown inFIG. 14.

DETAILED DESCRIPTION OF THE INVENTION

The reluctance transducer will be described in view of severalembodiments. The invention, however, is not limited to theseembodiments.

In FIG. 2 a first embodiment of the reluctance transducer is shownschematically in cross section. The shape of the soft ferromagnetic yoke(2) of the transducer (1) has some similarity with a horseshoe, definingan intermediate space (5) between the two ends (3,4) of the yoke forreceiving the soft ferromagnetic core element (6). The yoke has two endportions, a first end portion (15) with the first yoke end (3) and asecond end portion (16) with the second yoke end (4). The core elementand the yoke are movable relative to each other in a direction indicatedby the arrow. It is noted that a movement in a perpendicular directionmay be prevented or allowed. In the latter case the movement of the coreelement may be a movement in the combined horizontal en verticaldirection. To allow a movement in the direction of the arrow, viz. inthe direction from the first end (3) of the yoke to the second end (4)of the yoke and vice versa, there are movement spaces at each side ofthe core element. A first movement space (13) is present between thefirst end (3) of the yoke and the core element. A second movement space(14) is present between the second end (4) of the yoke and the coreelement.

The yoke and the core element are soft ferromagnetic which means thatthey have the property to conduct a magnetic flux. Soft ferromagneticmaterials can be magnetised but do not remain magnetised permanentlyafter removal of the cause of magnetisation. Although the yoke and thecore element of the reluctance transducer can be made out of any softferromagnetic material, they are preferably made of ferromagneticmaterials selected from the group consisting of iron, cobalt, nickel,and steel or made of materials mainly made out of these ferromagneticmaterials.

The two permanent magnets (7,8) are arranged opposite to each other atthe inner side of the yoke such that the two magnets define a space (10)for receiving the movable core element. The magnets are arrangedparallel to the end portions (15,16) of the yoke and are facing eachother with equivalent magnetic poles (N1,N2), for example the Northpoles. So the magnets are oriented in opposite direction along a commonline through the magnetic axis of the magnets. The poles (N1, N2) aremagnetically insulated from each other by the intermediate space (10),which space is partially filled by the core element. To allow movementof the core element in the direction of the arrow, there is a firstmovement space (11) between the core element and the first magnet (7)and a second movement space (12) between the core element and the secondmagnet.

The other poles (S1,S2) of the two magnets, for example the South poles,are mechanically and magnetically coupled to each other via anintermediate soft ferromagnetic element (9), in this embodiment being aportion of the yoke. This soft ferromagnetic element with its lowmagnetic resistance allows the magnetic flux to pass. The magnets (7,8)are arranged relative to the end portions (15,16) of the yoke such thatthere is a space (17,18) between each magnet and the corresponding endportion of the yoke to avoid a short circuiting of the magnetic field.Preferably the distance between the magnets and the end portions is muchlarger than the distance between the core element and the magnets.

The core element is adapted to be movable in the space that is formed bythe intermediate space (10) between the permanent magnets and theintermediate space (5) between the ends of the yoke in a direction fromthe first end (3) of the yoke to the second end (4) of the yoke and viceversa. So, the thickness of the core element, the distance between thetwo ends of the yoke, and the distance between the magnets is such thatthere is sufficient room for movement of the core element. In apreferred embodiment the distance between the ends of the yoke and thedistance between poles (N1,N2) of the magnets is the same. Further, theintermediate space (10) between the permanent magnets may be locatedmainly in line with the intermediate space (5) between the ends of theyoke, i.e. the ends (3,4) of the yoke may be positioned on substantiallythe same offset position as the facing end portions of the poles (N1,N2) of the corresponding permanent magnets (7,8). In such an embodimentthe thickness of the core element, which may be a disc or plate, may beuniform. Alternatively, the intermediate space (10) between thepermanent magnets may be staggered relative to the intermediate space(5) between the ends of the yoke. Further, the distance between thepoles (N1,N2) and the ends (3,4) may be different and the thickness ofthe core element may be different at different position, for examplebetween the magnets and between the yoke ends. Then, the core elementmay have a non-uniform contour, e.g. having a staggered profile.

In the lateral direction, viz. in the direction perpendicular to thearrow, the core element is sufficiently long to extend at leastpartially between both the permanent magnets and the ends of the yoke toallow magnetic flux of both magnets going through the core element. Whenreference is made to a moving or movable core element, it is understoodthat this includes situations in which the yoke or the magnets aremoving or movable relative to the core element.

This embodiment of the transducer can be applied for stiffnesscompensation because of its negative stiffness. However, by applying awinding at a portion of the yoke such an embodiment of the transducercan be applied as a magnetic bearing, an actuator, or as a sensor.

FIG. 3 shows an embodiment of the reluctance transducer (20) in whichthe permanent magnets (7,8) are arranged different from the embodimentin FIG. 2. Here the permanent magnets are arranged at the periphery ofthe yoke. The embodiment of the reluctance transducer (20) shown in FIG.3 comprises an electrical winding and can be applied as a magneticbearing, as an actuator or as a sensor. The winding (23) is wound arounda portion (19) of the yoke such that an electrical current in thewinding will induce a magnetic flux (I) in the soft ferromagnetic yoke.Each of the two end portions of the yoke splits up in two branches. Oneof the ends (3,4) at each end portion defines an intermediate space (5)similar to the embodiment of FIG. 2. At the other branches (25,26), thetwo permanent magnets (7,8) are arranged such that the ends (3,4) of theyoke and the free ends of the magnets define an intermediate space forreceiving the soft ferromagnetic core element (6), which element ismovable in a direction from the first end (3) of the yoke to the secondend (4) of the yoke and vice versa. It is appreciated that theconsiderations concerning the position of the magnets relative to theyoke and the dimensions of the core element given for the embodiment ofFIG. 2 also apply for this embodiment.

Also shown in FIG. 3 are two magnetic flux circuits (II, III), one fromeach of the permanent magnets. Circuit (II) comprises the firstpermanent magnet (7) and circuit (III) comprises the second permanentmagnet (8). The direction of the magnetic flux (21) of the firstpermanent magnet (7) at the first end (3) is opposite to the directionof the magnetic flux (22) of the second permanent magnet (8) at thesecond end (4) of the yoke. In FIG. 3, the South poles of the permanentmagnets are facing each other and are insulated from each other by thespace (10) which is partly filled by the core element (6). The Northpoles are mechanically and magnetically coupled by a soft ferromagneticportion (9) of the yoke. The permanent magnets may also be arranged suchthat the North poles are facing each other and the South poles arecoupled magnetically by a portion of the soft ferromagnetic yoke. Foreach of the permanent magnets there is an additional circuit of whichonly the circuit (IV) comprising the first magnet is shown.

It is noted that the magnets in the embodiment of FIG. 3 may be arrangedat different positions to obtain a similar magnetic circuit. As anexample, one such an embodiment is shown in FIG. 4. The yoke of thisreluctance transducer comprises three separate soft ferromagneticportions. The first portion (32) has a form similar to a horse shoe. Anelectrical winding (23) is wound around a part (19) of this portion ifthe transducer is applied as a magnetic bearing, as an actuator or as asensor. As mentioned before, this winding may be omitted for applicationas stiffness compensation. The two ends (3,4) of this portion of theyoke leave an intermediate space (5) for the movable soft ferromagneticcore element (6). A second soft ferromagnetic portion (31) of the yokeis mechanically coupled with the first portion by the first permanentmagnet (7). The third soft magnetic portion (33) of the yoke ismechanically coupled with the first portion by the second permanentmagnet (8) and separated from the second portion by a space (34) that ispartly filled by the soft ferromagnetic core element. The first magnetand the second magnet are arranged such that the direction (21) of themagnetic flux of the first magnet (7) at the first end (3) of the yokeis opposite to the direction (22) of the second magnet (8) and thesecond end (4). The same holds mutatis mutandis for the flux at the freeends (37,38) of the second and third portion of the yoke. There, theflux from the first magnet (7) is directed downwards and the directionof the flux from the second magnet (8) is directed upwards. The coreelement (6) is situated in the space formed by the space between the twoends (3,4) of the first portion of the yoke and the space between thefree ends (37,38) of the second and third soft ferromagnetic portion ofthe yoke, respectively.

The embodiments shown in the FIGS. 3 and 4 are symmetric with respect toa horizontal line through the core element and the electrical winding.It is noted that several other arrangements of winding and magnets arepossible. The winding may for example be wound around the upper or lowerpart of the yoke. Further, an embodiment that is a hybrid of theembodiment shown in FIGS. 3 and 4 is possible. The first magnet (7) canbe arranged relative to the yoke as shown in FIG. 3, while the secondmagnet (8) is arrange in the yoke as shown in FIG. 4.

FIG. 5 shows a cross-sectional view of a preferred embodiment of thereluctance transducer (40), which can be applied as a magnetic bearing,as an actuator or as a sensor. In this embodiment of the reluctancetransducer, the two ends (3,4) of the soft ferromagnetic yoke (2) areprovided with a cavity (41,42) in which a permanent magnet (7,8) isarranged. The cavities may be as deep as the length of the magnets thuscreating a flat surface of magnet and yoke at the sides facing the coreelement (6). However, the magnets may also be longer such that theyprotrude from the ends of the yoke. The magnets may also be shorter suchthat there is a deepening at the position of the magnets. The magnetsare arranged opposite to each other such that the equivalent poles(N1,N2) are facing each other. In FIG. 5, it are the North poles but itmay also be the South poles of the permanent magnets. The magnets aremechanically and magnetically coupled to the yoke at the equivalentpoles (S1,S2) and a portion (9) of the yoke couples the two equivalentpoles magnetically. Between the inner side walls of the cavities (41,42)and the magnets (7,8) there is a space (17,18), Preferably this space,more in particular the distance between the magnets and the side wallsof the cavities, is much larger than the distance between the otherequivalent poles (N1,N2) and the core element.

The magnets and the ends of the yoke define a space (5) in which themovable core element (6) is situated. More in particular the spaceallows a movement in the direction from the first end (3) of the yokeand the first magnet (7) to the second end (4) of the yoke and thesecond magnet (8) as indicated by the arrow. Preferably, the coreelement is only movable in the vertical direction, viz. a directionparallel to the magnetic axis of the two magnets. Mechanical measuresmay be taken to prevent the core element from moving in the horizontaldirection, viz. perpendicular to the direction of the magnetic axis.Such measures may also be omitted. Due to the magnetic forces exerted onthe core element by the permanent magnets, the reluctance transducer hasthe characteristics of a negative stiffness. A negative stiffnesscancels or reduces the stiffness of a system in which it is placed byexerting an opposing force. When during use of the reluctance transducerthe distance (13) between the core element (6) and the first end (3) ofthe yoke, and the distance (11) between the core element and the firstmagnet (7), increases than the distance between the core element and thesecond end (4) of the yoke and between the core element and the secondmagnet decreases.

The electrical winding (23) that is wound around a portion (19) of thesoft ferromagnetic yoke can be used to induce a magnetic flux in thesoft ferromagnetic yoke by applying a voltage across the winding tocreate an electrical current in the winding. This additional magneticflux, viz. in addition to the flux of the two permanent magnets, causesan additional force on the core element (6). As a consequence the coreelement will move towards the first end (3) or the second end (4) of theyoke. This allows applying the reluctance transducer as an actuator. Ifthe electrical winding is omitted than the reluctance transducer can beapplied for stiffness compensation.

The reluctance transducer shown in FIG. 5 can also be applied as asensor for measuring a displacement, a velocity, or an acceleration. Adisplacement of the core element will result in a voltage across thewinding and an electrical current in the winding, depending on thedisplacement.

In the embodiments of the reluctance transducer discussed so far thepermanent magnets are mechanically coupled to each other via a portionof the soft ferromagnetic yoke. In the embodiment of the reluctancetransducer (50) of which a cross-sectional view is shown in FIG. 6, themagnets are mechanically and magnetically coupled via a softferromagnetic portion (54) of the core element (51). An advantage ofthis embodiment of the reluctance transducer is that the point at whichthe magnetic force grips the core element does not change relative tothe centre of gravity of the core element when the core element moves inlateral direction, viz. in a direction perpendicular to the directionfrom the first end (3) of the yoke to the second end (4) of the yoke.The core element is provided with two cavities for receiving thepermanent magnets. One cavity (53) is arranged at the side of theelement that is facing the first end (3) of the soft magnetic yoke andthe other cavity (52) is arranged at the side facing the second end (4)of the yoke. Each of the cavities holds a permanent magnet that is fixedat one pole to the bottom of the cavity. The inner dimensions of thecavities in the lateral direction are larger than the width of themagnets to obtain a space between the magnets and the side walls of thecavities. More in particular, the space between the magnets and the sidewalls of the cavities is sufficient to force the magnetic flux of thefirst magnet (7) mainly via the first slot (13) and the magnetic flux ofthe second magnet (8) mainly via the second slot (14).

The two permanent magnets (7,8) are arranged relative to each other sothat equivalent poles (S1,S2) that are mechanically and magneticallycoupled by a soft ferromagnetic portion (54) of the core element, arefacing each other and thus their magnetic orientation is opposite. Theother two equivalent poles (N1,N2) are magnetically insulated from eachother by the spaces (13,14) that allow the core element moving from thefirst end (3) of the yoke to the second end (4) of the yoke, and viceversa. Instead of being arranged in cavities, the magnets may forexample also be arranged at the opposite surfaces of a flat coreelement. In FIG. 6 a electrical winding (23) for applying the reluctancetransducer as a magnetic bearing, a sensor or as an actuator is shown,but such a winding can be omitted when the reluctance transducer isapplied for stiffness compensation.

The embodiment of the reluctance transducer (60) shown in FIG. 7illustrates that the direction of the movement of the soft ferromagneticcore element need not to be the direction of the axis of the permanentmagnets. Although there are many similarities between the embodimentsshown in FIG. 3 and FIG. 7, there also differences that may beadvantageous for specific applications. The soft ferromagnetic coreelement (6) is situated in the space that is defined by two ends (3,4)of the yoke and the magnets (7,8). The two magnets are arranged along acommon line but the first magnet and the second magnet are magneticallyoriented in opposite direction so that the equivalent poles, here theNorth poles, are facing each other. The magnetic axis of the magnets isdirected perpendicular to the direction from the first end(3) of theyoke to the second end (4) of the yoke. The North poles of the magnetsare magnetically insulated from each other by a space comprising thecore element (6).

The other equivalent poles, here the South poles, are magnetically andmechanically coupled by a soft ferromagnetic portion (9) of the yoke.The static forces on the core element are such that they try to minimizethe magnetic resistance of the magnetic circuit of which the coreelement is a part of. The electrical winding (23) can induce a magneticflux as indicated by the circuit (I), the flux passing the core elementin the direction from the first end (3) of the yoke to the second end(4) of the yoke, or vice versa. The second permanent magnet (8) is partof a magnetic circuit (III), whereas the first permanent magnet (7) ispart of another the magnetic circuit (II). The first and second magnetare also part of a magnetic circuit comprising a part of the portion (9)of the yoke that couples the South poles of the two magnets. Only themagnetic circuit (IV) comprising the first magnet is indicated in FIG.7.

The core element can be moved from the first end (3) to the second end(4) of the yoke and visa versa by a magnetic flux (III) that is inducedby the winding (23). In stead of applying this reluctance transducer asan actuator, it can also be applied as a sensor by measuring the currentin the winding or the voltage across the winding. As in otherembodiments of the reluctance transducer, the winding can be omitted forstiffness correction.

The above-described reluctance transducer can be used in a wide varietyof applications, including magnetically driven actuators, magneticsensors sensing mechanical displacements, magnetic bearings, andcompensation structures for reducing a positive stiffness or increasinga negative stiffness of a structure.

Another embodiment of the reluctance transducer is discussed withreference to FIG. 8. As the embodiment shown in FIG. 3, the yoke of theembodiment (70) of the reluctance transducer shown in FIG. 8 comprisesthree separate soft ferromagnetic portions. The first portion has a formthat has some similarities with the yoke shown in FIG. 7 and has a shapethat can be described as a horseshoe of which the end parts are split intwo branches. The end (3,4) of one of the two branches is facing thecore element (6), whereas at the other branch a magnet is arranged. Anelectrical winding (23) is wound around a part of this first portion ifthe transducer is applied as a magnetic bearing, as a sensor, or as anactuator. The winding can be omitted when the transducer is applied asnegative stiffness for stiffness compensation. The first magnet (7) andthe second magnet (8) are arranged such that the two permanent magnetsare in parallel but in opposite direction, viz. they have an oppositemagnetic orientation relative to each other. The direction of themagnetic axis of the two magnets is mainly in parallel with thedirection of movement of the core element as indicated by the arrow inFIG. 8. The ends of the two branches (71,73) facing the core element areeach facing an opposite side of the core element.

A second soft ferromagnetic portion (71) of the yoke is mechanicallycoupled to the first portion by the first permanent magnet (7). A thirdsoft ferromagnetic portion (8) of the yoke is mechanically coupled tothe first portion by the second permanent magnet (8). The second andthird portion of the yoke have a U-shape and are arranged such that theend (74) of the second portion is facing one side of the core element,whereas the end (75) of the third portion is facing the other side ofthe core element. The portions of the yoke are arranged such that theends (3,4,74,75) leave a space for the movable core element. The end ofthe branch (71) that is mechanically coupled to the first magnet (7) andthe second end (4) of the yoke are facing the same side of the coreelement. The other side of the core element is facing the end of the ofthe branch (73) that is mechanically coupled to the second magnet (8)and the first end (3) of the yoke.

Magnetic circuits similar to those of the reluctance transducer shown inFIG. 8 can be realised by arranging the three portions of the yoke andthe magnets in a different way. Without being exhaustive, some of suchalternatives are mentioned here. The first magnet (7) and the secondyoke portion (71) may change place such that the magnet is facing thecore element with its South pole (and changing the shape of the secondportion accordingly). The second magnet(8) may be placed at the positionof the upper part of third portion, with its South pole directed to theleft. Of course the shape of the third portion and the first portionhave to be adapted such that space for the core element remains thesame.

Preferably, the magnets of the embodiment of the reluctance transducershown in the FIGS. 2, 3, 5, and 6 are arranged such that the magneticaxes of both magnets are on the same line. However, the axes of themagnets may also be shifted relative to each other. This means that, forexample, the first magnet (7) in the afore mentioned figures may bearranged more to the left.

In all the embodiments discussed above, the orientation of the magnetsmay be reversed, so the North poles may be changed into South poles andvice versa provided that both magnets (7,8) are reversed simultaneously.

FIG. 9 shows a schematic perspective view of an actuator structure (100)comprising a frame (110) supporting an array of individual actuators(101-105) that are based on the basic reluctance transducer principleexplained referring to the embodiment shown in FIG. 5. Preferably, theframe (110) is composed of material with a low relative magneticpermeability, viz. a non-magnetic material. As an example, the frame(110) is manufactured from aluminum. The array of actuators (101-105) isarranged as a straight row. However, the array of actuators can alsoextend in two dimensions, forming a two-dimensional grid of actuators,e.g. for controlling the position and/or orientation of mirror elementsin an adaptive mirror. Further, the actuators may have different lengthand/or may be arranged at different levels in the direction of the thirddimension, for example for optimal actuation of mirror elements. One ofthe actuators (101) is shown in partially cross-sectional view in FIG. 9and will be further discussed with reference to FIG. 10.

FIG. 10 shows a schematic exploded view of an actuator (101) from theactuator structure (100) shown in FIG. 9. The soft ferromagnetic yokehas a first portion (111) including the first end (3) and the second end(4), respectively. Further, the soft ferromagnetic yoke has a secondportion (112) including the section that is surrounded by the electricalwindings (23), the first and the second portion (111, 112), the upperand lower lid (116,117) forming a magnetically closed loop bridging theintermediate space (5).

The first portion (111) is implemented as a cylinder shell. Further, thesecond portion (112) includes a cylinder kernel, concentrically arrangedwith respect to the cylinder shell (111). The second portion alsoincludes intermediate sections magnetically connecting axial ends (112a,b, 111 a,b) of the cylinder kernel (112) and the cylinder shell (111),respectively. The intermediate sections are implemented as an upper lid(116) and a lower lid (117). Thus, the cylinder shell (111) bridging theintermediate space (5), the upper lid (116), the cylinder kernel (112)and the lower lid (117) constitute a circuit for conducting a magneticflux. The cylinder kernel (112) may protrude from the upper lid (116)and/or the lower lid (117). Electrical windings (23) are located betweenthe cylinder kernel (112) and the cylinder shell (111). The electricalwindings (23) are provided with electrical terminals (23 a,b) to feedthe windings (23).

The construction shown in FIG. 10 thus includes a cylinder kernel (112),electrical windings (23) and a cylinder shell (111) arrangedconcentrically with respect to each other. The actuator (101) alsoincludes permanent magnetic elements (109 a,b, 110 a,b) as explained inmore detail referring to FIG. 10.

FIG. 11 shows a schematic side view of a detail of the actuator (101)shown in FIG. 10. A first end (123) of the first portion (111) of thesoft ferromagnetic yoke includes a cavity wherein a first permanentmagnet (109 a) is received. Similarly, a second end (124) of the firstportion (111) of the soft ferromagnetic yoke includes a cavity wherein asecond permanent magnet (110 a) is received. The first and secondpermanent magnets (109 a, 110 a) face each other and are orientedmagnetically in opposite order, i.e. with the North pole facing eachother, or with the South pole facing each other.

A core (126) is integrally manufactured with the frame (110), and isformed as a lever pivoting with respect to an elastic hinge (127).Further, the actuator (101) includes a separate actuator element (128)extending axially with respect to the cylinder shell (111) and cylinderkernel (112), through the frame (110) and protruding from a top sectionof the frame, see also FIG. 9. The core partially occupies theintermediate space (5) between the ends (123, 124) of the softferromagnetic yoke. Between the core (126) and the ends (123, 124) ofthe soft ferromagnetic yoke, a first and second slot (13,14),respectively, is defined allowing the core to move upwardly anddownwardly to drive the separate actuator element (128).

By flowing an electrical current through the electrical windings (23),the core (126) moves upwardly or downwardly, depending on the flowingdirection in the electrical windings. In principle, the configurationcan also be used for sensing a mechanical displacement. When the core(126) moves, an electrical current is forced to flow through theelectrical windings (23). The amount of electric current is a measurefor the displacement of the core (126).

The left-hand part of FIG. 11 shows an actuator (101) wherein thepermanent magnets (109, 110) are present. The right-hand part of FIG. 11shows an actuator (102) wherein the magnets are not yet installed, priorto a final manufacturing stage. The integrally formed core (126) can berealized by a wire cut electrical discharge machining process. Anadvantage of using such a machining process is that the space may bemade with tolerances that are smaller than the tolerances obtainable byan assembling process. However, also other manufacturing processes areapplicable.

As shown in FIG. 10, two pairs of permanent magnet pairs are provided,the permanent magnets (109 a, 110 a; 109 b, 110 b) of each individualpair being arranged opposite to each other near the first and secondslot (13,14), respectively, and magnetically oriented opposite to eachother, thereby enhancing the power of the actuator. Generally, even morethan two pairs of permanent magnets can be implemented. In the shownembodiment, the magnetic elements (109, 110) are located at the sameposition relative to the hinge (127) so that the magnetic forces addconstructively. In principle, other locations can be selected, therebyenabling movements of the core in mutually different directions.

FIG. 12 shows a schematic perspective view of an embodiment of amagnetic bearing (200) including a reluctance transducer. The magneticbearing (200) essentially blocks movement of a core (201), also calledrotor, in a specific direction, here the x-direction. The bearing (200)includes a soft ferromagnetic yoke (202) and electric windings (215),also called a coil, for generating a magnetic flux in the softferromagnetic yoke (202). According to an aspect of the currentinvention, the soft ferromagnetic yoke (202) includes first and secondends (203, 204) that are formed as mutually aligned bars. In the shownembodiment, the rotor (201) is provided with a pair of magnetic elements(209, 210), located and oriented opposite to each other.

FIG. 13 shows a schematic cross-sectional view of the magnetic bearing(200) shown in FIG. 12. By locating the magnetic elements (209, 210) onthe rotor (201), the points of application of the magnetic force remaininvariant. Therefore, a movement of the rotor (201) in a directiontransverse to the x-direction does not influence the magneticperformance of the bearing (200). The effective flux path (F) is notchanged during a movement of the rotor.

FIG. 14 shows a schematic perspective view of another embodiment of amagnetic bearing (300). Here, an additional reluctance transducer isprovided blocking the rotor (301) in a further direction, thez-direction. The magnetic bearing (300) includes two coils (310, 311)wherein the ends (303, 304) of the first coil (310) face to a direction,the x-direction, that is different from the direction, i.e. thez-direction, to which the ends (323, 324) of the second coil (311) face.In the shown embodiment, the facing directions are transverse withrespect to each other. However, in principle, also other directions arepossible, e.g. a tilted direction. Further, the transducer (300)comprises two pairs of permanent magnetic elements (340 a, 340 b; 341 a,341 b), see FIG. 15, the permanent magnets of each individual pair beingarranged opposite to each other near the first and second slot,respectively, of corresponding coil ends and magnetically orientedopposite to each other, in conformity with the above-describedprinciple.

The magnetic bearing (300) shown in FIG. 15 has a dual characterblocking movement of the rotor (301) in two directions, i.e. in thex-direction and in the z-direction. In principle, a further dualmagnetic bearing can be applied to the rotor (301) blocking two furtherdegrees of freedom, in terms of location and/or orientation. Inaddition, a further single magnetic bearing can be added to thestructure, e.g. a magnetic bearing as shown in FIG. 12. Then, only asingle degree of freedom is left to the rotor (301), e.g. a degree offreedom in shifting in a particular direction.

FIG. 15 shows a schematic exploded view of the magnetic bearing (300)shown in FIG. 14. As shown, the soft magnetic elements can bemanufactured from distinct ferromagnetic material layers (380) tocounteract eddy current effects. This also applies to the soft magneticmaterial on the core (301).

It is noted that, generally, the core includes soft magnetic elementsfor magnetically conducting the flux bridging between the ends of thesoft magnetic elements. It is further noted that a multiple number ofelectrical windings can be applied to a soft magnetic element. Also,resistive elements can be added in the electrical windings.

The invention is not restricted to the embodiments described herein. Itwill be understood that many variants are possible.

Other such variants will be apparent for the person skilled in the artand are considered to fall within the scope of the invention as definedin the following claims.

1. A reluctance transducer comprising: a soft ferromagnetic yoke havinga first end and a second end, the first end and the second end definingan intermediate space, a soft ferromagnetic core element partly fillingthe intermediate space, the core element and the yoke being movablerelative to each other in a direction between the first end of the yokeand the second end of the yoke, a first permanent magnet and a secondpermanent magnet arranged relative to each other such that the firstmagnet and the second magnet exert opposite forces on the core element,and wherein a first pole of the first magnet is mechanically andmagnetically coupled by an intermediate soft ferromagnetic element to anequivalent pole of the second magnet, wherein a second pole of the firstmagnet is magnetically insulated from an equivalent pole of the secondmagnet.
 2. The reluctance transducer according to claim 1, wherein thefirst permanent magnet and the second permanent magnet are arranged suchthat the direction of a magnetic flux of the first permanent magnet atthe first end of the yoke is opposite to the direction of a magneticflux of the second permanent magnet at the second end of the yoke. 3.The reluctance transducer according to claim 1, wherein the intermediatesoft ferromagnetic element is a portion of the yoke.
 4. The reluctancetransducer according to claim 3, wherein the first end of the softferromagnetic yoke is provided with a first cavity receiving the firstpermanent magnet and the second end of the soft ferromagnetic yoke isprovided with a second cavity receiving the second permanent magnet. 5.The reluctance transducer according to claim 2, wherein the yokecomprises a first portion fixed to the first pole of the first magnetand a second portion fixed to the second pole of the first magnet. 6.The reluctance transducer according to claim 5, comprising a thirdportion of the yoke and wherein the first pole of the second magnet isfixed to the third portion of the yoke and the second portion is fixedto the second pole of the second magnet.
 7. The reluctance transduceraccording to claim 2, wherein the soft ferromagnetic elementmechanically coupling the two permanent magnets is a portion of the softferromagnetic core element.
 8. The reluctance transducer according toclaim 7, wherein a first surface of the core element is facing the firstend of the yoke and a second surface of the core element is facing thesecond end of the yoke and wherein the first surface is provided with acavity receiving the first magnet and the second surface is providedwith a cavity receiving the second magnet.
 9. The reluctance transduceraccording to claim 3, wherein the direction from the first end of theyoke to the second end of the yoke is perpendicular to the directionfrom the first permanent magnet to the second permanent magnet.
 10. Thereluctance transducer according to claim 1, wherein the softferromagnetic yoke has a first portion comprising the first end and thesecond end, and a second portion that is surrounded by an electricalwinding.
 11. The reluctance transducer according to claim 4, wherein thesoft ferromagnetic yoke has a first portion implemented as a cylindershell comprising the first end and the second end, and a second portionthat is surrounded by an electrical winding, and wherein the secondportion includes a cylinder kernel, concentrically arranged with respectto the cylinder shell, the second portion further including intermediatesections magnetically connecting axial ends of the cylinder kernel andcylinder shell, respectively.
 12. The reluctance transducer according toclaim 4, comprising a multiple number of permanent magnetic pairs, thepermanent magnetic elements of each individual pair being arrangedopposite to each other near first and second slots, respectively, andmagnetically oriented opposite to each other.
 13. The reluctancetransducer according to claim 4, wherein the core element is formed as alever moving an actuator element or a sensor element.
 14. The reluctancetransducer according to claim 4, wherein first and second end portionsof the soft magnetic yoke are formed as mutually aligned bars.